The objectives of this research are to delineate the intrathymic events in the acquisition of T-cell immunocompetence by stem cells. We will focus on the question of what steps are involved in "thymus derivation" of lymphocytes, what changes occur in surface topography in the course of a stem cell traversing the thymus, what recondition phenomena are involved that commit a precursor cell to thymus derivation, to establish by histological and ultrastructural means whether there is direct cell to cell interaction between the thymus epithelium and stem cells and if this occurs by means of transient gap junctions. To do this we have purified thymic epithelium rendering it free from fibroblasts and are studying the interaction of bone marrow and thymic precursor cells with the epithelium.